They're using the Craigslist reputation and page views indirectly. The information is only on Craigslist because people placed it there, and what was their reason for choosing Craigslist in the first place? Reputation and page views.
By that same argument, Google News is using the reputation and page views of The New York Times and The Wall Street Journal. Google as a whole is using the reputation and page view of the entire web.
